During combustion of fuels that contain sulfur compounds, oxides of sulfur (SOX), such as sulfur dioxide (SO2), and sulfur trioxide (SO3) are produced as a result of oxidation of the sulfur. Some fuels may contain nitrogen compounds that contribute to the formation of oxides of nitrogen (NOX), which are primarily formed at high temperatures by the reaction of nitrogen and oxygen from the air used for the reaction with the fuel. These reaction compounds, SOX and NOX, are reported to form acids that may contribute to “acid rain.” Federal and state regulations dictate the amount of these and other pollutants, which may be emitted. The regulations are becoming more stringent and plant operators are facing greater difficulties in meeting the regulatory requirements. Many technologies have been developed for reduction of SOX and NOX, but few can remove both types of pollutants simultaneously in a dry process or reliably achieve cost effective levels of reduction.
In the past to meet the regulatory requirements, coal-burning power plants have often employed a scrubbing process, which commonly uses calcium compounds to react with SOX to form gypsum. This waste product is normally discarded as a voluminous liquid slurry in an impoundment and ultimately is capped with a clay barrier, which is then covered with topsoil once the slurry is de-watered over time. Alternatively, some power-plant operators have chosen to burn coal that contains much lower amounts of sulfur to reduce the quantities of SOX emitted to the atmosphere. In the case of NOX, operators often choose to decrease the temperature at which the coal is burned. This in turn decreases the amount of NOX produced and therefore emitted; however, low temperature combustion does not utilize the full heating value of the coal, resulting in loss of efficiency.
Turbine plants normally use natural gas, which contains little or no sulfur compounds, to power the turbines, and therefore virtually no SOX is emitted. On the other hand at the temperature that the turbines are commonly operated, substantial NOX is produced. In addition to Selective Catalytic Reduction (SCR) processes for conversion of NOX to nitrogen, water vapor, and oxygen, which can be safely discharged, some operators choose to reduce the temperature at which the turbines are operated and thereby reduce the amount of NOX emitted. With lower temperatures the full combustion/heating value of natural gas is not realized, resulting in loss of efficiency. Unfortunately for these operators, newer environmental regulation will require even greater reduction of SOX and NOX emissions necessitating newer or more effective removal technologies and/or further reductions in efficiency.
Operators of older coal-burning power plants are often running out of space to dispose of solid wastes associated with use of scrubbers that use calcium compounds to form gypsum. Operators of newer plants would choose to eliminate the problem from the outset if the technology were available. Additionally, all power plants, new and old, are faced with upcoming technology requirements of further reducing emissions of NOX and will have to address this issue in the near future. Thus, plants that currently meet the requirements for SOX emissions are facing stricter requirements for reduction of NOX for which there has been little or no economically feasible technology available.
The nitrogen oxides, which are pollutants, are nitric oxide (NO) and nitrogen dioxide (NO2) or its dimer (N2O4). The relatively inert nitric oxide is often only removed with great difficulty relative to NO2. The lower oxide of nitrogen, nitrous oxide (N2O), is not considered a pollutant at the levels usually found in ambient air, or as usually discharged from air emission sources. Nitric oxide (NO) does however; oxidize in the atmosphere to produce nitrogen dioxide (NO2). The sulfur oxides considered being pollutants are sulfur dioxide (SO2) and sulfur trioxide (SO3).
Typical sources of nitrogen and sulfur oxide pollutants are power plant stack gases, automobile exhaust gases, heating-plant stack gases, and emissions from various industrial process, such as smelting operations and nitric and sulfuric acid plants. Power plant emissions represent an especially formidable source of nitrogen oxides and sulfur oxides, by virtue of the very large tonnage of these pollutants and such emissions discharged into the atmosphere annually. Moreover, because of the low concentration of the pollutants in such emissions, typically 500 ppm or less for nitrogen oxides and 3,000 ppm or less for sulfur dioxide, their removal is difficult because very large volumes of gas must be treated.
Of the few practical systems, which have hitherto been proposed for the removal of nitrogen oxides from power plant flue gases, all have certain disadvantages. Various methods have been proposed for the removal of sulfur dioxide from power plant flue gases, but they too have disadvantages. For example, wet scrubbing systems based on aqueous alkaline materials, such as solutions of sodium carbonate or sodium sulfite, or slurries of magnesia, lime or limestone, usually necessitate cooling the flue gas to about 55° C. in order to establish a water phase. At these temperatures, the treated gas requires reheating in order to develop enough buoyancy to obtain an adequate plume rise from the stack. U.S. Pat. No. 4,369,167 teaches removing pollutant gases and trace metals with a lime slurry. A wet scrubbing method using a limestone solution is described in U.S. Pat. No. 5,199,263.
Considerable work has also been done in an attempt to reduce NOX pollutants by the addition of combustion catalysts, usually organo-metallic compounds, to the fuel during combustion. However, the results of such attempts have been less successful than staged combustion. NOX oxidation to N2 is facilitated by ammonia, methane, et al. which is not effected by SOX is described in U.S. Pat. No. 4,112,053. U.S. Pat. No. 4,500,281 teaches the limitations of organo-metallic catalysts for NOX removal versus staged combustion. Heavy metal sulfide with ammonia is described for reducing NOX in stack gases in U.S. Pat. No. 3,981,971.
Many fuels, and particularly those normally solid fuels such as coal, lignite, etc., also contain substantial amounts of bound or fuel sulfur with the result that conventional combustion produces substantial amounts of SOX pollutants which are also subject to pollution control. It has generally been the opinion of workers in the art that those conditions employed in staged combustion, particularly two-stage rich-lean combustion for NOX reduction, will likewise lower the level of SOX emissions. However, it has been found that little or no reduction in SOX emissions can be obtained in a two-stage, rich-lean combustion process. Indeed, it has been found that the presence of substantial amounts of sulfur in a fuel also has a detrimental effect on NOX reduction in a two-stage, rich-lean process.
Considerable effort has been expended to remove sulfur from normally solid fuels, such as coal, lignite, etc. Such processes include wet scrubbing of stack gases from coal-fired burners. However, such systems are capital intensive and the disposal of wet sulfite sludge, which is produced as a result of such scrubbing techniques, is also a problem. Cost inefficiencies result from the often-large differential pressures across a wet scrubber removal system; differential pressures in excess of 30 inches of water column (WC) are not unusual. Also, the flue gases must be reheated after scrubbing in order to send them up the stack, thus reducing the efficiency of the system. Both U.S. Pat. Nos. 4,102,982 and 5,366,710 describe the wet scrubbing of SOX and NOX.
In accordance with other techniques, sulfur scavengers are utilized, usually in fluidized bed burners, to act as scavengers for the sulfur and convert the same to solid compounds which are removed with the ash. The usual scavengers in this type of operation include limestone (calcium carbonate) and dolomite (magnesium-calcium carbonate) because of availability and cost. However, the burning techniques are complex and expensive to operate and control; and the burner equipment is comparatively expensive. Dissolving coal or like material in a molten salt compound is described in U.S. Pat. No. 4,033,113. U.S. Pat. No. 4,843,980 teaches using alkali metal salt during the combustion of coal or other carbonaceous material with further efficiency by adding a metal oxide. A sulfur scavenger added upstream to a combustion zone is described in U.S. Pat. No. 4,500,281.
The combustion gas stream from a coal-burning power plant is also a major source of airborne acid gases, fly ash, mercury compounds, and elemental mercury in vapor form. Coal contains various sulfides, including mercury sulfide. Mercury sulfide reacts to form elemental mercury and SOX in the combustion boiler. At the same time other sulfides are oxidized to SOX and the nitrogen in the combustion air is oxidized to NOX. Downstream of the boiler, in the ducts and stack of the combustion system, and then in the atmosphere, part of the elemental mercury is re-oxidized, primarily to mercuric chloride (HgCl2). This occurs by reactions with chloride ions or the like normally present in combustion reaction gases flowing through the combustion system of a coal-burning power plant.
Many power plants emit daily amounts of up to a pound of mercury, as elemental mercury and mercury compounds. The concentration of mercury in the stream of combustion gas is about 4.7 parts per billion (ppb) or 0.0047 parts per million (ppm). Past efforts to remove mercury from the stream of combustion gas, before it leaves the stack of a power plant, include: (a) injection, into the combustion gas stream, of activated carbon particles or particulate sodium sulfide or activated alumina without sulfur; and (b) flowing the combustion gas stream through a bed of activated particles. When activated carbon particle injection is employed, the mercuric chloride in the gas stream is removed from the gas stream in a bag house and collected as part of a powder containing other pollutants in particulate form. Mercuric chloride and other particulate mercury compounds that may be in the gas stream can be more readily removed from the gas stream at a bag house than can elemental mercury. Activated carbon injection for mercury removal along with an activated particle bed is described in U.S. Pat. No. 5,672,323.
When the gas stream flows through a bed of activated carbon particles, mercury compounds are adsorbed on the surface of the activated carbon particles and remain there. Elemental mercury, usually present in vapor form in combustion gases, is not adsorbed on the activated carbon to any substantial extent without first being oxidized into a compound of mercury. U.S. Pat. No. 5,607,496 teaches the oxidation of mercury and subsequent absorption to particles and utilization of alumina are described therein.
Sodium sulfide particle injection can be utilized to form mercuric sulfide (HgS), which is more readily removable from the gas stream at a bag house than is elemental mercury. The conversion of mercury to a sulfide compound with subsequent capture in a dust separator is detailed in U.S. Pat. No. 6,214,304.
Essentially, all of the above techniques create solid waste disposal problems. The solids or particulates, including fly ash, collected at the bag house and the spent activated carbon removed from the bed of activated carbon, all contain mercury compounds and thus pose special problems with respect to burial at landfills where strictly localized containment of the mercury compounds is imperative. The concentration of mercury compounds in particulates or solids collected from a bag house is relatively minute; therefore, a very small quantity of mercury would be dispersed throughout relatively massive volumes of a landfill, wherever the bag house solids or particulates are dumped. Moreover, with respect to activated carbon, that material is relatively expensive, and once spent activated carbon particles are removed from an adsorbent bed, they cannot be easily regenerated and used again.
In the activated alumina process, mercury compounds in the gas stream can be adsorbed and retained on the surface of activated particles, but much of the elemental mercury will not be so affected. Thus elemental mercury in the combustion gas stream is oxidized to form mercury compounds (e.g. mercuric chloride), and catalysts are employed to promote the oxidation process. However, such processes do not capture SOX and NOX.
The use of oxides of manganese to remove sulfur compounds from gas streams is known in the art. Oxides of manganese are known to form sulfates of manganese from SOX and nitrates of manganese from NOX when contacted with a gas containing these pollutants. U.S. Pat. No. 1,851,312 describes an early use of oxides of manganese to remove sulfur compounds from a combustible gas stream. U.S. Pat. No. 3,150,923 describes a dry bed of oxides of manganese to remove SOX. A wet method to remove SOX with oxides of manganese is described in U.S. Pat. No. 2,984,545. A special filter impregnated with manganese oxide to remove totally reduced sulfur compounds is described in U.S. Pat. No. 5,112,796. Another method in U.S. Pat. No. 4,164,545 describes using an ion exchange resin to trap the products of manganese oxide and SOX and NOX. The use of certain types of oxides of manganese to remove SOX is disclosed U.S. Pat. Nos. 3,723,598 and 3,898,320. Some of the known methods of bringing oxides of manganese in contact with a gas stream, i.e., sprayed slurries, beds of manganese ore or special filters, have been cumbersome. Although the prior art teaches the use of oxides of manganese to remove SOX and/or NOX, they do not teach an adaptable system or process that can capture SOX and/or NOX and other pollutants with oxides of manganese and monitor and adjust system operational parameters, such as differential pressure, to provide real-time system control.
Bag houses have traditionally been used as filters to remove particulates from high volume gas streams. U.S. Pat. No. 4,954,324 describes a bag house used as a collector of products generated through the use of ammonia and sodium bicarbonate to remove SOX and NOX from a gas stream. U.S. Pat. No. 4,925,633 describes a bag house as a site of reaction for SOX and NOX with the reagents, ammonia and alkali. U.S. Pat. No. 4,581,219 describes a bag house as a reactor for highly efficient removal of SOX only with a calcium-based reagent and alkaline metal salt. Although these prior art discloses and teach the use of bag houses for removal of particulates and as a reaction chamber, they do not teach the use of bag houses in an adaptable system capable of monitoring and adjusting system operational parameters, such as differential pressure, to capture SOX and/or NOX and other pollutants with oxides of manganese.
In view of the aforementioned problems of known processes for removal of SOX, NOX, mercury compounds, and elemental mercury as well as other pollutants from combustion gases, process gases, and other industrial waste gases, it would be desirable to provide a dry process for removal of SOX and NOX as well as other pollutants from a gas stream. It is further desirable to have a dry removal process that eliminates the environmental impacts of the disposal of large volumes of mercury containing solids and particulates and significant amounts of gypsum generated during SOX wet removal processes.
Wet removal processes can result in significant differential pressures across a removal system. Differential pressures above 30 inches of water column have been observed in wet removal processes. Such large differential pressures are costly because significant energy must be expended to counter the differential pressure and provide a waste gas stream with sufficient energy to flow up and out of a stack. A system and process that can accomplish pollutant removal with minimal or controlled differential pressure across the system therefore would be desirable and cost effective for most industry sectors processing or emitting significant amounts of combustion gases, process gases, and other industrial gases.
The calcium compounds utilized in SOX wet scrubbing methods form gypsum in the process. They are purchased and consumed in significant quantities and once gypsum is formed the calcium compounds cannot be recovered, at least not cost-effectively. Thus, it would be desirable to have a removal method employing a sorbent that not only can remove pollutants from a gas stream but that can be regenerated, recovered, and then recycled or reused for removal of additional pollutants from a gas stream.
To realize such a system and process, it would need to incorporate process controls and software that can monitor and adjust operational parameters from computer stations onsite or at remote locations through interface with a sophisticated electronics network incorporating an industrial processor. This would allow a technician to monitor and adjust operational parameters in real-time providing controls of such operational parameters as system differential pressure and pollutant capture rates or removal efficiencies. Such a network would be desirable for its real-time control and off-site accessibility.
In light of increased energy demand and recent energy shortages, it would be desirable to be able to return to operational utility idled power plants that have been decommissioned because their gypsum impoundments have reached capacity. This could be accomplished with retrofits of a system employing a regenerable sorbent in a dry removal process that does not require the use of calcium compounds. Such a system would also be readily adapted and incorporated into new power plants that may be coming on line. Utility plants and independent power plants currently in operation could readily be retrofitted with such a system. Further, such a system could be of significant value in enabling emissions sources to comply with emission standards or air quality permit conditions. With the reductions in emissions of pollutants such as NOX and SOX, marketable emissions trading credits could be made available or non-attainment areas for state or national ambient air quality standards may be able to achieve attainment status. Such scenarios would allow for development in areas where regulatory requirements previously prohibited industrial development or expansion.
The systems and processes of the present invention in their various embodiments can achieve and realize the aforementioned advantages, objectives, and desirable benefits.